Method for epitaxially growing thin films

ABSTRACT

A monocrystalline material of the formulation Hg(1 x)Cd(x)Te is grown epitaxially on a seed or substrate monocrystal of Cd Te, or the like. The reactants are mixed in the vapor phase and held at a temperature which prevents binary combinations. The ternary vapor phase mixture is then rapidly cooled to supersaturation and condensed on the seed crystal substrate. In a dynamic system, the mercury vapor acts as a carrier gas as well as a reactant.

, United States Patent [72] Inventors Donald R. Carpenter Vestal;

Gerald W. Manley, Johnson City, N.Y.; Philip S. McDermott, Athens, Pa.;Ralph J.

Riley, Apalachin, N.Y. [2]] Appl. No. 763,307 [22] Filed Sept. 27, I968[45] Patented Nov.9,l97l

[73] Assignee InternationalBusiness Machines Corporation Armonk, N.Y.

[5 4] METHOD FOR EPlTAX-IALLY GROWING THIN FILMS 12 Claims, 6 DrawingFigs.

[52] U.S. Cl

[51] Int. Cl ..C23c 13/08, B05c 11/14, C23c 13/00 [50] Field ofSearch117/201, 106 A;23/209; 252/623; 148/174, 175

[56] References Cited UNITED STATES PATENTS 2,759,861 8/1956 Collins117/106 X 2,938,816 5/1960 Gunther.... 117/201 X 2,994,621 8/1961 Hugleet al. 117/201 3,394,390 7/1968 Cheney 23/209X Primary Examiner-WilliamL. Jarvis Attorney-Hanifin and Jancin ABSTRACT: A monocrystallinematerial of the formulation Hg Cd Te is grown epitaxially on a seed orsubstrate monocrystal of Cd Te, or the like. The reactants are mixed inthe vapor phase and held at a temperature which prevents binarycombinations. The ternary vapor phase mixture is then rapidly cooled tosupersaturation and condensed on the seed crystal substrate. In adynamic system, the mercury vapor acts as a carrier gas as well as areactant.

REGULATED 21 RE POWER SUPPLY POWER SUPPLY AIR 000mm souRct PAIENTEDunv9197: 3,619,283

SHEET 1 [1F 2 DONALD R. CARPENTER GERALD W. MANLEY PHILIP S. McDERMOTTRALPH J. RILEY ATTOR EY PAIENTEDunv 9 ml SHEET 2 BF 2 FIG. 3

Hg comm GROWN LAYER (MOL 1) FIG. 6

FIG. 5

ITTO

Hg OVERPRESSURE (IN ATMOSPHERES) SUBSTRATE TEMP C METHOD FOR EPITAXIALLYGROWING THIN FILMS BACKGROUND OF THE INVENTION The present inventionrelates to the formation of semiconductor bodies by vapor deposition andparticularly to a method and apparatus for epitaxially vapor growingthin film materials of mercury, cadmium, and tellurium.

DESCRIPTION OF THE PRIOR ART In prior processes for producingcrystalline film materials from vapors of the elements and compounds ofmercury, cadmium and tellurium, as well as other II-VI elements, it wasthought that the presence of other vapor reactants were required toobtain growth and stoichiometric ratio control. In such processes it wascommon to include halogen or halogen compound vapors which produced achemical disproportionation reaction and which acted as a transportagent for the vapor phase reactants to the growth site on a seed crystalor substrate. In such processes, the transport agent vapor condensed tosome degree with the principal reactants. This tended to contaminate theend product film resulting in loss of purity and prevented theproduction of films having precise stoichiometric ratios.

SUMMARY OF THE INVENTION The broad object of the present invention is toprovide an improved process for expitaxially growing films from vaporsof elements and compounds of materials in the II-VI valence groups.

It is a specific object to provide a process for epitaxially growingthin film crystals having the formula Hg Cd Te where x is greater thanzero and less than 1, and which has greater purity and homogeneity.

It is a further specific object to provide a process for growingmonocrystalline epitaxial films of very precise stoichiometric ratiofrom vapors of mercury, cadmium, and tellurium elements or compounds.

The above, as well as other objects of this invention are readilyachieved by vaporizing the film producing reactant materials inpredetermined stoichiometric quantities in a chamber devoid of any otherreactants. In the specific system for making epitaxial films havingternary combinations of mercury, cadmium, and tellurium, the mercuryvapor, in addition to being a combining reactant of the end productfilm, also serves as the transport agent. In carrying out the process,the vapors of mercury, cadmium, and tellurium are mixed in a reactionchamber and maintained in the vapor phase at temperatures whicheffectively prevent preferential binary combinations. This mixture isthen rapidly cooled proximate the growth site to the point ofsupersaturation causing a ternary reaction to occur whereby the filmgrown has the same stoichiometric ratio established by the compositionratio in the vapor phase. By eliminating the halogens or otherdisproportionation reactant vapors, and by using the vapor of one of theconstituent reactants, namely mercury, as the carrier, epitaxial filmswere obtained having superior intrinsic properties. Preferably, theprocess is carried out dynamically. This is done by flowing mercuryvapors through successive zones of an evacuated chamber wherevaporization of the other reactants occurs and to the deposition site.Mercury is vaporized from a liquid source and excess mercury iscondensed adjacent the condensation reaction region and returned to theliquid source.

The mercury recycling and redistribution acts to dynamically flow themercury through the reaction chamber causing the reactant mixture tomove more rapidly through the mixing and cooling zones to the growthsite.

To achieve this dynamic system for epitaxially growing Hg Cd Te film,the furnace apparatus is equipped with a separate recirculating conduitconnected to opposite ends of the reaction chamber where vaporgeneration, mixing and film growing take place in a multitemperaturezone, A supply of mercury is provided in liquid form in the returnconduit. Heating means is provided to vaporize the mercury forintroduction at the upstream end of the reaction chamber. Individuallycontrollable heating means is provided at successive regions along thereaction chamber for volatilizing source materials of cadmium, andtellurium, and in the region surrounding the growth site. Cooling meansat the growth site is used to control the temperature of a vapor growingsubstrate and in combination with the heating means establishes a sharptemperature gradient in the vicinity of the substrate to provide rapidcoolmg.

The foregoing and other objects, features and advantages of theinvention will be apparent from the following more particulardescription of preferred embodiments of the invention, as illustrated inthe accompanying drawings.

BRIEF DESCRIPTION OF THE DRAWINGS FIG. 1 is a cross-sectional view of anapparatus for epitaxially growing thin films in accordance with thepresent invention;

FIG. 2 is a cross-sectional view of a portion of the deposition sitemechanism showing the means for cooling a deposition substrate;

FIG. 3 is an isometric view of the cooling cap of FIG. 2, showing thedetail structure for using thermocouple temperature measurement;

FIG. 4 is an isometric view of the cooling probe of FIG. 2;

FIG. 5 is a graph showing the operational conditions for epitaxiallygrowing films according to this invention; and

FIG. 6 is a three-dimensional graph for showing the operationalconditions for the practice of the present invention.

DESCRIPTION OF THE PREFERRED EMBODIMENTS Referring to FIGS. 1-4, a filmgrowing apparatus comprises a cylindrical quartz reaction chamber 10closed at one end by a transparent quartz wall 11 fused to the innerwall of chamber 10. The other end of the chamber 10 is inwardly taperedto receive a quartz stopper l2 outwardly tapered to coact with thechamber to produce an airtight taper joint 13. The stopper 12 has ahollow inwardly extending finger 14 to which is attached a quartzcylindrical cap 15. A substrate crystal 16 is held tightly in positionagainst the cap 15 by a spring clip 17 which preferably is an integralpart of cap 14. While only one clip 17 is shown, additional clips aswell as other means may be provided, if necessary, to insure goodthermal contact of the substrate 16 with cap 15. Intermediate the endsof chamber 10 are ports 18 and 19 which are connected to form a closedloop with chamber 10. When stopper 12 is in position, finger 14 extendsinwardly to a position slightly beyond port 18. The chamber is evacuatedby means of suitable vacuum system 21 connected through pipe 22 openedand closed by valve 23.

The reaction chamber 10 is designed to have separately controlledheating zones A, B, and C, and these are provided by suitable inductionwindings 24, 25, and 26 connected to suitable regulated power supplies27, 28, and 29, respectively. A fourth heating means comprises inductioncoil 30 wound on conduit 20 proximate inlet port 19 and connected to aseparate regulated power supply 31.

Means for temperature regulating the substrate 16 comprises metalliccylindrical heat sink 32 located within the hollow core of finger 14 andmeans for supplying cooling air thereto which comprises a hollow metalprobe 33 connected by pipe 34 to an air coolant source 35. The probe 33and pipe 34 are designed to be readily removable from finger 14 in orderthat stopper 12 may be removed from chamber 10 in preparation for thevapor growth operation. As best seen in FIGS. 2 and 4, the heat sink 32,when in position at the far end of finger 14, makes surface contact withthe probe finger 14 and through cap 15 to substrate 16. The coolingprobe 32 has a conical point 36 with plural apertures 37. When inposition, the point of cylinder 36 makes contact with the inside of heatsink 32, and when heat sink 32 is made preferably of silver, the

point penetrates heat sink 32 thereby increasing the surface contact andholding the probe more firmly in place. Air flowing from coolant source35, as depicted by arrows 38, exits from holes 37 into contact with theinterior surfaces of heat sink 32. Cooling also is obtained byconduction from heat sink 32 to probe 33. Temperature measurementincludes a thermocouple 39, or the like, with the junction locatedwithin aperture 40 of heat sink 32 and connected to indicator controlmeans 41. When in position, the junction of thermocouple 39 is incontact with the interior wall offinger 14.

In the operation of the reaction apparatus of F IG. 1, mercury liquid 42is placed in conduit 20 at a level whereby liquid mercury is within theheating zone of winding 30. Source material 43 and 44 in boats 45 and46, respectively, are placed in zones A and B, respectively.

A substrate 16, is selected to grow the desired film monocrystal andafter cleaning is placed in position on finger l4 and held there by clip17. Stopper 12 is then inserted into the end of chamber and maintainedin place by suitable means (not shown) to maintain an airtight seal 13.Probe 33 is then inserted in finger 14 to make contact with heat sink 32and connection by pipe 34 to coolant 35 is then completed. Upon closingof chamber 10 by stopper 12, vacuum source 21 is operated, with valve 23open, to effect initial pumpdown of chamber 10 to remove undesirablecontaminants such as water and oxygen in chamber 10. If desirable, thesubstrate 16 may be further cleaned by backetching. In the back-etchprocess, the substrate 16 is heated by coil 26 to drive off any cleaningsolvent as well as some of the constituent materials of the substrate16. These materials may be also removed by the evacuation means 21. Theinitial pumpdown is preferably performed after vaporization of mercuryhas been initiated. When the desired vacuum pressure is reached, valve23 is closed and vacuum system 21 shut down. The heating coils 24, 25,and 26, as well as coolant source 35, are activated to begin the filmgrowth process.

In general, the growth process is practiced by heating the liquidmercury 42 causing mercury vapor to enter reaction chamber 10 throughinlet port 19. Likewise, the temperatures in zones A and B are set topredetermined heat levels to volatilize the reactants from sourcematerials 43 and 44. Due to the lower temperature at the substrate 16,caused by coolant from source 35 and in the region of finger 14, atemperature differential exists in the chamber 10 which causes mercuryvapor to flow from port 19 through zones A and B, thereby producing amixture of mercury vapor and gases of sources 43 and 44. Due to the samepressure drop, the gaseous mixture flows to zone C where the coil 26 isset to establish the temperature which prevents binary combinations inthe vapor phase of the gaseous reactants. With coolant from source 35,the temperature substrate is maintained at a temperature low enough tocause the constituents of the gaseous mixture to supersaturate andcondense on the surface ofsubstrate 16.

In accordance with the present invention, the substrate temperature 16and the temperature of the gaseous mixture in zone C are maintained atlevels which provide a very sharp temperature gradient in the vicinityof the surface of substrate 16. A temperature in zone C of at least 50C. above the substrate temperature should be maintained. Thus, thegaseous mixture experiences rapid cooling in the region close to thesubstrate 16 and becomes supersaturated causing the constituentreactants to condense without the aid of additional disproportionationreactants ordinarily used in such vapor growing. Throughout the entiregrowing process, the mercury vapor acts as a transport agent and sweepsthe other reactants through the zones A-C to region of deposition. Thisprevents back diffusion of other reactants to port 19 and thus preventscontamination of the supply of mercury 42. The portion of the materialthat does not react at the growing surface is carried out of thedeposition region of zone C and deposits on the walls of chamber 10 inthe vicinity of port 18. The mercury vapor condensed in this regionflows through port 18 back to the supply 42. Thus. the apparatusoperates dynamically and functions much as a mercury diffusion pumpthereby providing a high degree of control over the process whileassuring maximum purity of the mixtures to thereby achieve stoichiometrywithout contamination.

In practicing the present invention to produce an epitaxial film, thesubstrate 16 is a monocrystal carefully selected and prepared fordeposition. The substrate 16 would be a material having a latticespacing similar to that of the film to be grown. The substrate 16 is cutfrom monocrystal ingots previously aligned by X-ray techniques toprovide a growth receiving surface along a predeterminedcrystallographic plane. The receiving surface of substrate 16 is thenpolished and cleaned.

The source materials 42 and 43 are selected from suitable crystallinematerials, or the like, which are preferably pulverized to a fine degreeto obtain maximum volatilization when heated. If ingot materials areused, polishing and cleaning may likewise be performed to minimize theamount ofcontaminants introduced into the atmosphere of chamber 10.

The following are specifications of one example of the preferred form ofthe present invention:

1. The substrate 16 was a Cd Te monocrystal cut from an ingot along theplane. The growth receiving surface of substrate 16 was ground andpolished with one-fourth micron alumina, then chemically etched in abromine alcohol solution for 2 minutes; followed by copious alcoholrinse with subsequent drying.

. The source material 40 located in zone A was approximately 2 gramscadmium, ground and pulverized to a fineness of 50 mesh. The sourcematerial 41 in zone B was approximately 2 grams of tellurium ground andpulverized to a fineness of 50 mesh.

3. Heat coil 30 was energized to a temperature of approximately 300 C.to generate mercury vapor and the vacuum system 21 then operated toproduce initial pumpdown. When the pressure in chamber 10 reached l0Torr., valve 23 was closed and system 21 shut down. Power supply 27 and28 were then turned on to heat coil 24 and 25 to produce a zone Atemperature of 440 C., and a zone B temperature of 520 C. At the sametime, power supply 29 was operated to produce a zone C temperature of460 C. Coolant was supplied from source 35 to provide a substrate 16temperature of 280 C. Measurement of the zone and temperatures wasdetermined by appropriately placed thermocouples using a potentiometerrecorder. For a typical run time of l.252 hours under the aboveconditions, a stoichimetric growth layer of 7-10 mils thickness wasproduced whose composition was Hg 0.8 Cd 0.2 Te.

At the end of the growing process, the power to the heating coils 24,25, and 26 is discontinued to allow cooling. The supply of coolant airis also discontinued. This prevents growth at the substrate. Aftercooling to room temperature the stopper may be removed without oxidationoccurring on the film.

Other examples of samples and process conditions, as well as results,are set forth in the following table.

MRM-IM 280 450 2 Hr. 2 mils While the above examples are specificillustrations of actual samples, it will be appreciated that otherformulations may be devised in a wide range of temperatures andconcentrations in accordance with the principles of the invention. FIG.5 shows that acceptable growth of stoichiometric film may be obtainedwhere substrate temperatures vary within the range of 200 to 350 C. upto a range of 400 to 600 C. for a mercury overpressure of from 1 to 100plus atmospheres. Outside of the region bounded by curves 45 and 46 thegrown film no longer has a stoichimetric composition and becomes analloy.

Other variations possible for practicing the present invention areillustrated in FIG. 6 which shows the relationship between Hg content inthe grown layer to the source temperature and substrate temperature forthe required molar quantity of Te. Stoichiometry is obtained in mercurycadmium telluride films for any set of conditions which fall on thecurved surface 47.

While the above examples illustrate specific substrate materials forgrowth purposes, other crystalline materials may be used, such asHg'l'e, Pb'le, SnTe, or the like.

Also, while the specific examples show stoichiometric film growth fromelemental mercury, cadmium, and tellurium, other ternary systems in thelI-Vl groups could be used, such as Zn Cd Te, Zn Hg Te, Hg Cd Se, Zn HgSe, Zn Cd Se, and the like, where Zn and Hg are the transporting agents.

While the invention has been particularly shown and described withreference to preferred embodiments thereof, it will be understood bythose skilled in the art that the foregoing and other changes in formand details may be made therein without departing from the spirit andscope of the invention.

We claim:

1. A process for the vapor growing of ternary films comprised ofelements selected from the ll-Vl valence groups comprising:

forming a ternary gaseous mixture consisting of the vapors of saidelements in a chamber substantially devoid of contaminant elements;

maintaining said ternary gaseous mixture at a reaction temperature whichprevents preferential binary chemical combinations in said vapor phasewhereby said mixture has a composition ratio in the vapor phase at leastequal to the desired stoichiometric ratio of a film to be grown; and

rapidly cooling said mixture to cause supersaturation and condensationof said elements on a growth substrate in the same stoichiometric ratioof said elements in said vapor phase.

2. A process in accordance with claim 1 in which said elements aremercury, cadmium, and tellurium, and said stoichiometric ratio isdefined by the expression Hg Cd Te where x is greater than zero and lessthan one.

3. A process in accordance with claim 2 in which said mercury, cadmium,and tellurium are separately volatilized, and said mercury vapor acts asa transport agent for said cadmium and tellurium vapors.

4. A process in accordance with claim 3 in which said mixture ismaintained in the region proximate the deposition substrate at atemperature of at least 50 C. above the temperature of the substrate.

5. A process in accordance with claim 3 in which said cadmium vapor isgenerated from a source heated to a temperature of 440 C., saidtellurium vapor is generated from a source heated to a temperature of520 C., said mercury is volatilized with an overpressure of 30 microns,said reaction temperature of the mixture in the region proximate saidsubstrate is 460 C. and said substrate temperature is approximately 280C. whereby said growth film is Hg Cd Te.

6. A process in accordance with claim 3 in which said mercury vapor iscaused to mix successively with vapors of cadmium and tellurium and tocarry said mixture to a condensation reaction and vapor growth zone. I

7. A process in accordance with claim 6 in which said mixing is effectedby flowing a stream of mercury vapor unidirectionally from a mercuryvaporization source through the vaporization sources of said cadmium andtellurium to said growth site.

8. A process in accordance with claim 7 in which said unidirectionalflow is effected by controlling the temperature and pressure of theconstituent vapors at the successive vaporization sources and at thegrowth site.

9. A process in accordance with claim 8 in which said growth sitetemperature and the temperature of the vapor phase mixture is controlledto regulate the stoichiometric ratio ofsaid mixture and said film.

10. A process in accordance with claim 7 in which an excess of mercuryvapor is supplied to said stream, and excess mercury remaining from thecondensation reaction is collected and returned to said source of saidmercury vapor.

11. A process in accordance with claim 10 in which said mercury vapor isgenerated from a supply of liquid mercury, and said excess mercury isdistilled and returned to said liquid supply.

12. A process in accordance with claim 7 in which said mercury vapor isgenerated from a liquid supply, and excess mercury is condensed outsidethe growth site and returned to said liquid supply.

Patent No. o 51 q 922 Inventor(s) jnvnmhpr 9 .L 1971 Donald R.Carpenter, Gerald W. Manley, mott and Ralnh J. Rilev It is certifiedthat error appears in the above-identified patent and that said LettersPatent are hereby corrected as shown below:

On the cover sheet, in the "ABSTRACT", line 2,

the formula reading reading Hg Cd Te should read Hg Cd Te Column 1, line32 the formula reading Claim 2, Column 6, line 3, the formula Hg Cd 'I'eshould read Hg Cd Te Signed and sealed this 18th day of July 1 972.

SEAL) Attest:

EDWARD M.FLE'ICHER,JR. Atte sting Officer ROBERT GOTTSCHALK Commissionerof Patents

2. A process in accordance with claim 1 in which said elements aremercury, cadmium, and tellurium, and said stoichiometric ratio isdefined by the expression Hg(1 x)Cd(x)Te where x is greater than zeroand less than one.
 3. A process in accordance with claim 2 in which saidmercury, cadmium, and tellurium are separately volatilized, and saidmercury vapor acts as a transport agent for said cadmium and telluriumvapors.
 4. A process in accordance with claim 3 in which said mixture ismaintained in the region proximate the deposition substrate at atemperature of at least 50* C. above the temperature of the substrate.5. A process in accordance with claim 3 in which said cadmium vapor isgenerated from a source heated to a temperature of 440* C., saidtellurium vapor is generated from a source heated to a temperature of520* C., said mercury is volatilized with an overpressure of 30 microns,said reaction temperature of the mixture in the region proximate saidsubstrate is 460* C. and said substrate temperature is approximately280* C. whereby said growth film is Hg0.8 Cd0.2 Te.
 6. A process inaccordance with claim 3 in which said mercury vapor is caused to mixsuccessively with vapors of cadmium and tellurium and to carry saidmixture to a condensation reaction and vapor growth zone.
 7. A processin accordance with claim 6 in which said mixing is effected by flowing astream of mercury vapor unidirectionally from a mercury vaporizationsource through the vaporization sources of said cadmium and tellurium tosaid growth site.
 8. A process in accordance with claim 7 in which saidunidirectional flow is effected by controlling the temperature andpressure of the constituent vapors at the successive vaporizationsources and at the growth site.
 9. A process in accordance with claim 8in which said growth site temperature and the temperature of the vaporphase mixture is controlled to regulate the stoichiometric ratio of saidmixture and said film.
 10. A process in accordance with claim 7 in whichan excess of mercury vapor is supplied to said stream, and excessmercury remaining from the condensation reaction is collected andreturned to said source of said mercury vapor.
 11. A process inaccordance with claim 10 in which said mercury vapor is generated from asupply of liquid mercury, and said excess mercury is distilled andreturned to said liquid supply.
 12. A process in accordance with claim 7in which said mercury vapor is generated from a liquid supply, andexcess mercury is condensed outside the growth site and returned to saidliquid supply.